Fluid properties measurements using planar Quasi-Scholte waveguides

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Abstract

Interface acoustic waves can propagate in solid-solid and solid-liquid interfaces.
They are perturbed by ambient conditions, interface materials and characteristics. The
wave energy are shared between two interfacing media, with its distribution depending
on these properties. In this study, it was aimed to make liquid characterization (density)
using interface acoustic waves. The waves propagating between the plate and the liquid
interface are kind of flexural waves and known as quasi-Scholte mode.
Firstly, the wave propagation for multilayered system is analytically modelled. A
global matrix method is employed for obtaining the dispersion characteristics of the
multilayer structure by considering the boundary conditions of the system.
In order to develop the dipstick sensor, liquid characterization is performed firstly
by test cell method for longitudinal bulk velocity of the liquid with known density. Test
cells with two different lengths were used for measurements. In this technique,
transmitting and receiving transducers are placed on opposite sides of test cell. Based on
the distance between transducers and time of flight, velocity of longitudinal bulk waves
was calculated. Mixtures of ethanol-pure water were prepared at various ratios and
measurements were taken at different temperatures.
Quasi-Scholte mode dipstick sensoring is employed by a simple measurement
waveguide or dipstick inserted into the fluid to measure the velocity of the quasi-Scholte
mode on a plate. This method, is based on mode conversions of waves. When the shear
wave is excited on the aluminum plate, which is used as dipstick, Lamb waves occur at
the solid-gas surface. The lamb waves are then converted into different modes in the solidliquid
interface. Leaky Rayleigh and Scholte propagating modes exist in immersed part
as well. Quasi-Scholte wave propagate on the interface while the Leaky Rayleigh modes
lose their energy. By means of dipstick method, alcohol water mixtures which is used in
test cell method, were measured at room temperature.
The signals obtained as a result of the experiments were processed by the zero
phase slope method. Time delay between reflected signals are found from phase slopes
of the signals.